1. Field of the Invention
The present invention relates to a method for stably transferring a Bloch line which is present in a magnetic wall of a stripe magnetic domain.
The present invention also relates to a magnetic memory apparatus which can record and reproduce information stably and at a high speed by transferring a Bloch line which is an information carrier, by a novel method.
2. Related Background Art
Computer external memory, electronic file memory and still image file memory use memory devices such as magnetic tape, Winchester disks, floppy disks, optomagnetic disks or magnetic bubble memories. These memory devices other than the magnetic bubble memory, require relative motion between a record medium such as a tape or disk and a reproducing head. As a result, for high density recording, problems of tracking, drive and, wear of the medium and the head, dust, vibration, and, in the optical disk and the opto magnetic disk, a problem of focusing, occur.
As disclosed in U.S. patent application Ser. No. 434,538, filed Nov. 8, 1989, assigned to the assignee of the present invention, the magnetic bubble memory does not need a mechanical drive and is highly reliable. Accordingly, it has been considered that the magnetic bubble memory was advantageous in high density recording. However, since the magnetic bubble memory uses, as a bit, a circular magnetic domain (bubble) developed in a magnetic garnet film having an easy axis of easy magnetization which is normal to the film plane, an upper limit of recording density is several tens of M bits/chip even if a minimum bubble (0.3 .mu.m in diameter) restricted by the material properties of a currently available garnet film, is used. As compared with a semiconductor memory which will be a competing memory in future, there is not much difference in capacity. Accordingly, an application range as a memory is very narrow.
A Bloch line memory disclosed in U.S. Pat. No. 4,583,200 has recently been accorded attention as a way to overcome the limitation of recording density in the magnetic bubble memory. Technology relating to the Bloch line memory are disclosed in U.S. patent application Ser. No. 660,260 filed Feb. 26, 1991 and allowed Jul. 9, 1991, and U.S. Pat. Nos. 4,974,201 and 4,974,200.
In the Bloch line memory, two transition regions in which directions of magnetization twist in a magnetic wall which is present around a stripe magnetic domain developed in a magnetic garnet film are opposite to each other, that is, two regions (Bloch line) defined by a Nail magnetic wall between the Bloch magnetic wall structures are used as a unit of memory, and the pair of Bloch lines are used as one bit. In the Bloch line memory, since the width of a Bloch line is usually one tenth of the width of a magnetic domain, a recording density which is almost ten times as high as that of the magnetic bubble memory can be attained. For example, when a garnet film having a bubble diameter of 0.5 .mu.m is used, a memory capacity of 1.6 G bits/chip is attainable.
In the Bloch line memory, it is necessary to establish a stable position of the Bloch line pair, which is a unit of memory, in the magnetic wall, to store information stably into the memory, and stably transfer it bit by bit at a high speed.
FIGS. 1A and 1B and FIGS. 2A and 2B show stabilization and transfer method for the Bloch line pairs shown in U.S. Pat. No. 4,583,200. FIG. 1A shows a stabilization pattern formed on a magnetic film to stabilize the Bloch line pair. FIG. 1B illustrates a potential in the magnetic film and stabilization of the Bloch line memory, due to the formation of the stabilization pattern. FIG. 2A shows a method for transferring the Bloch line pair and FIG. 2B shows a pulse waveform of a perpendicular pulse magnetic field used to transfer the Bloch line pair.
In the past, in order to stabilize the Bloch line pair, a stripe pattern 3 extending orthogonally to a longitudinal direction of the stripe magnetic domain 1 was formed on the magnetic film as shown in FIG. 1A, and crosspoints of the magnetic wall 2 of the stripe magnetic domain 1 and the stripe pattern 3 were used as the stable points of the Bloch line pair.
The stripe pattern 3 may be formed by a ferromagnetic material layer having an axis of any magnetization normal to a film plane or in the stripe magnetic domain 1 or an axis of easy magnetization along a width of the stripe in the film plane, or by implanting ions into the magnetic film to a uniform depth.
By providing a distribution 4 of a magnetic field H.sub.X along the longitudinal direction of the stripe magnetic domain 1 as shown in FIG. 1B, along the magnetic wall 2 of the stripe magnetic domain 1, by the ferromagnetic material film pattern or the ion implanted region pattern, a periodic potential well to the Bloch line pair 6 formed by the Sehmann energy between the magnetic field H.sub.X and the magnetization 7 acts as a magnetic wall between the Bloch line pair.
In the above memory, the magnetization of the Bloch line is rotated by a gyro-force by application of a pulse magnetic field HP.sub.P.perp. in a direction of arrow 8 normal to the film plane of the magnetic garnet film as shown in FIG. 2A. In FIG. 2A, the Bloch line is moved in a direction of arrow 9 as the magnetization is rotated so that the Bloch line pair 6 is transferred by one bit.
In this system, the shape of the potential well to the Bloch line pair is laterally symmetric. Thus, if the Bloch line transfer pulse magnetic field is of simple square waveform, the Bloch line pair is moved in the direction of the arrow 9 at the rise of the pulse but is returned to the original position at the fall of the pulse, and stable transfer in one direction is not attained. In the prior art, unidirectional transfer is attained by applying a pulse magnetic field which has a sufficiently longer fall time t.sub.2 than a rise time t.sub.1 as shown by graph 10 of FIG. 2B. However, in this system, since two Bloch lines are simultaneously moved, the Bloch lines vibrate due to attraction force or repelling force between the Bloch lines and stable transfer is not attained. Further, since a spacing between the Bloch lines of the pair is very short, the depth of the potential well due to the Sehmann energy is shallow and stabilization of the Bloch line pair 6 is hard to attain. When the magnetic garnet film is made of conventional bubble material, it is necessary that the fall time t.sub.2 is longer than 1 .mu. sec. This causes a very low Bloch line transfer speed. In order to generate the pulse waveform as shown in FIG. 2B, the required electric circuit is more complex and a power consumption is larger than those for a square wave pulse.
When the Bloch lines are transferred by the prior art transfer method to record or reproduce information to or from the Bloch line memory, it is difficult to accurately record or reproduce information because of instability of Bloch line transfer, that is, instability of information transfer.